1. Field of the Invention
This invention relates to a pyroelectric type infrared detector using pyroelectric elements that develop electric charges based on changes in the amount of incident infrared rays, and also to a method of producing the same.
2. Description of the Prior Art
FIG. 1 is a perspective view of a pyroelectric type infrared detector that motivated this invention. Referring to FIG. 1, the numeral 1 denotes a pyroelectric element that develops electric charges based on changes in the amount of incident infrared rays, said pyroelectric element 1 being made, e.g., of single crystals of lithium tantalate (LiTaO.sub.3) or lithium niobate (LiNbO.sub.3). This pyroelectric element 1 is polarized in advance in the direction of arrow P in FIG. 1 so that its front and back sides are positive and negative, respectively. The front side of the pyroelectric element 1 is formed with first and second surface electrodes 2a and 2b of the same area, respectively, spaced 0.1-1.0 mm apart. The first and second surface electrodes 2a and 2b are formed by vacuum deposition of a material having high infrared absorbability, such as nichrome or gold-black. On the other hand, the back surface of the pyroelectric element 1 is formed throughout with a back surface electrode 3, which is formed by vacuum deposition of the same material as that for the first and second front surface electrodes 2a and 2b.
The first and second front surface electrodes 2a and 2b are formed at one of their respective corners with first and second lead wire connection electrodes 4a and 4b, respectively, which are made of aluminum. The lead wire connection electrodes 4a and 4b have first and second lead wires 5a and 5b respectively connected thereto by ultrasonic wedge type bonding.
The pyroelectric element 1 formed with the first and second surface electrodes 2a and 2b and the back surface electrode 3, as described above, is supported on a metallic support block 6 made, e.g., of phosphor bronze or stainless steel. The metallic support block 6 has support portions 7 formed by acid etching at the four corners of its upper surface. The support portions 7 and the back surface electrode 3 are bonded together, e.g., by an epoxy type insulating adhesive agent 8. Therefore, the back surface electrode 3 is disposed in opposed relation to the support block 6.
In the arrangement shown in FIG. 1, since a space 9 is present between the pyroelectric element 1 and the support block 6, the pyroelectric element 1 is heat-insulated by the surrounding air, with the result that the efficiency of development of electric charges by the pyroelectric element 1 with respect to changes in the amount of incident infrared rays is greatly increased.
FIG. 2 shows an electric circuit including the infrared detector shown in FIG. 1. In FIG. 2, the infrared detector is represented by first and second capacitors 10a and 10b electrically connected in series. The numeral 11 denotes an impedance conversion circuit which, in view of the high resistance of the pyroelectric element 1, is incorporated to lower the resistance associated with an output terminal 12.
The impedance conversion circuit 11 comprises an input resistor 13 having a resistance value of 10.sup.9 -10.sup.11 .OMEGA., and an FET (field-effect transistor) 14, and an output resistor 15 having a resistance value of about 10K.OMEGA..
The infrared detector, which is constructed in the manner described above, is used as an intruder alarm that detects the approach or invasion of a human being and reports this information. In such use, for example as schematically shown in FIG. 3, the infrared detector is placed at the focus of a concave mirror 16 having on its reflecting surface a metal film having high infrared reflection coefficient, such as an aluminum film. If an intruder passes by as indicated by an arrow M, infrared rays from the intruder fall on the first front surface electrode 2a of the pyroelectric element 1, as indicated by an arrow I.sub.1, resulting in a change in the amount of incident infrared rays, so that an electric charge develops. As the intruder moves, infrared rays from the intruder fall on the second front surface electrode 2b of the pyroelectric element 1, as indicated by an arrow I.sub.2, resulting in a change in the amount of incident infrared rays, so that an electric charge develops. On the basis of the thus produced electric charges, a signal which can be used to report the invasion of the intruder appears at the aforesaid output terminal 12.
If the amounts of infrared rays incident on portions of the pyroelectric element 1 corresponding to the first and second front surface electrodes 2a and 2b change at the same time owing to changes in the ambient temperature, equal amounts of electric charges will be produced in the front surface electrodes 2a and 2b at the same time, but such electric charges will be produced in such a manner as to cancel each other. Thus, there is no possibility of an erroneous alarm being given by changes in the ambient temperature.
As shown in the circuit diagram of FIG. 4, in the case where the infrared detector comprises the first and second capacitors 17a and 17b electrically connected in parallel, it is necessary that on portions of the pyroelectric element 1 corresponding to the first and second front surface electrodes 2a and 2b, the latter is polarized in opposite directions (indicated by arrows P.sub.1 and P.sub.2). Otherwise, said electric charges produced by changes in the ambient temperature could not be cancelled by each other. In such case, the first and second lead wires 5a and 5b are connected to the gate of the FET 14, and with an electrically conductive adhesive agent used as the adhesive agent 8, the back surface electrode 3 is grounded through the adhesive agent 8 and support block 6.
To constitute such infrared detector, it is necessary to produce the pyroelectric element 1 that is polarized in opposite directions (P.sub.1 and P.sub.2) on portions of the pyroelectric element 1 corresponding to the first and second front surface electrodes 2a and 2b. Thus, this involves a troublesome operation and brings about a problem that the production cost of the infrared detector is very high.